1. Field of the Invention
The present invention relates to a method of and an apparatus for manufacturing a semiconductor device, and more particularly to a method of and an apparatus for manufacturing a semiconductor device which is suitable for a dynamic RAM having a stacked capacitor.
2. Description of the Background Art
In the art of dynamic RAM, as capacitor region and electric charges accumulated therein decrease with refinement of a circuit, degradation of reliability due to software error and the like becomes pronounced. As a solution therefor, a variety of capacitors having three-dimensional structure are put forth. Among them, a so-called stacked capacitor is the most widely used capacitor.
FIG. 7 is a sectional view of a structure of stacked capacitor cell. A pair of source-drain diffusion layers 5 are disposed in an active region which is an upper portion of a silicon substrate 1 isolated by a field oxide film 2. A gate electrode 3 of polysilicon is formed above a portion of the silicon substrate 1 between the source-drain diffusion layers 5. Between the gate electrode 3 and the silicon substrate 1, disposed is a gate oxide film 13. The gate oxide film 13, the source-drain diffusion layers 5 and the gate electrode 3 constitute a switching transistor.
One of the source-drain diffusion layers 5 is connected to a capacitor accumulation electrode 8 of polysilicon, and the electric charges are fed therefrom to a capacitor 100 through the switching transistor. The capacitor accumulation electrode 8 is insulated by the other of the source-drain diffusion layers 5, the gate electrode 3 and an interlayer oxide film 6.
The capacitor 100 includes the capacitor accumulation electrode 8 and a silicon nitride film 9, a silicon oxide film 10 and a capacitor counter electrode 11 which are stacked on the capacitor accumulation electrode 8 in this order. The silicon nitride film 9 and the silicon oxide film 10 serve as a capacitor dielectric film. The stacked structure, in which the silicon oxide film 10 is formed over the silicon nitride film 9, suppresses a leak current of the capacitor 100 and ensures a long lifetime.
At first, the silicon nitride film 9 is formed on the interlayer oxide film 6 and the capacitor accumulation electrode 8 by a low pressure CVD (chemical vapor deposition) method. Next, the silicon nitride film 9 is oxidized in pyrogenic manner to form the silicon oxide film 10 on the surface thereof. The capacitor counter electrode 11 is made of polysilicon.
The method of forming the silicon nitride film 9 will be discussed below in more detail. FIG. 8 is a sectional view of an apparatus used for forming a silicon nitride film by a batch-type low pressure CVD method. A quartz tube 21 of hollow cylindrical construction is heated by three heaters 22 having resistance heating system, to 700.degree. C. to 800.degree. C. generally in forming a silicon nitride film. Metallic flanges 23 are each provided at the front of and at the back of the quartz tube 21 for support and vacuum sealing. The quartz tube 21, which is vacuum-sealed by the flanges 23, is evacuated with a vacuum pump 24, thus keeping at the degree of vacuum of 1 mTorr (0.133 Pa).
Now, the operation of forming the silicon nitride film will be discussed. About 100 sheets of silicon substrates 20 are supported on a jig 26, called a boat, of quartz, being uniformly spaced in perpendicular to this page. The boat 26 for supporting the silicon substrates 20 is inserted into the quartz tube 21 in the atmosphere and the flanges 23 close the quartz tube 21. Next, the quartz tube 21 is evacuated by activation of a vacuum pump 24. After the silicon substrates 20 are sufficiently heated to a prescribed temperature, dichlorosilane (SiH.sub.2 Cl.sub.2) and ammonia (NH.sub.3) as source gas for forming a silicon nitride film are fed into the quartz tube 21 through the left flange 23 while being flow-controlled by a mass flow controller 25. The mass flow controller 25 controls the pressure of the quartz tube 21 to 0.3 Torr (39.9 Pa) in general.
The gas flows of dichlorosilane and ammonia are generally about 30 SCCM and 180 SCCM, respectively. The volume flow rate of ammonia to dichlorosilane is in the range of 4 to 10, which is disclosed in Japanese Patent Application Laid-Open Gazette 6-120157.
Under this condition, the silicon nitride film is deposited on the silicon substrates 20 by the low pressure chemical vapor deposition method. After that, the gas in the quartz tube 21 is replaced with the nitrogen gas and the pressure inside the quartz tube 21 recovers to the atmospheric pressure. The boat 26 for supporting the silicon substrates 20 is taken out from the quartz tube 21 in the atmosphere. The formation of the silicon nitride film carried out as above ensures uniform thickness and reproducibility of the silicon nitride film.
In the background-art semiconductor device, an increase in the amount of charges accumulated in a capacitor has been achieved by use of stacked capacitor and the like. With further refinement, however, it is difficult to ensure the increase in the amount of accumulated charges only by use of the stacked capacitor structure. Then, for the purpose of increasing capacitance of a capacitor to ensure more amount of charges to be accumulated in the capacitor, a capacitor dielectric film is becoming thinner.
That disadvantageously causes more intense electric field to be created on the capacitor dielectric film and an increase in leak current. Furthermore, time-varying dielectric breakdown lifetime is shortened and so on. Thus, it becomes difficult to ensure reliability.
In addition, the silicon nitride film 9 is easily formed on the capacitor accumulation electrode 8, but is hard to form on the interlayer oxide film 6. Accordingly, when the capacitor dielectric film becomes thinner, the silicon nitride film 9 formed on the interlayer oxide film 6 becomes much thinner. In this case, when the silicon nitride film 9 is oxidized through pyrogenic oxidation to form the silicon oxide film 10 thereon, the oxidizing agent goes through the silicon nitride film 9 and the interlayer oxide film 6, thus oxidizing the capacitor accumulation electrode 8 of polysilicon. In short, a problem that the masking effect to prevent oxidation becomes weaker, as a silicon nitride film 9 becomes thinner.